Encapsulated values were being written with a length.

By setting vl to the length of the value, I was overwriting the
correct value of "0xffffffff".
Add vtkDICOMValue methods that use count arguments.
Use size_t instead of unsigned int as interface type.

Using unsinged int is likely to cause many many compiler warnings,
because unsigned int is smaller than size_t and ptrdiff_t on 64 bit
systems.
Clean up vtkDICOMValue code, avoid unnecessary calcs.
Remove deprecated vtkDICOMValue methods from code.
Add another 8-bit field in vtkDICOMValue for overflow.

This is specifically for values that use delimiters instead of fixed
lengths, such as OB or UN VRs that store encapsulated data.  By adding
an 8-bit overflow byte for the 32-bit NumberOfValues field, it becomes
possible to store encapsulated data with sizes up to 2^40 bytes (1TB).
Values that use fixed lengths are still limited to 2^32-2 bytes (4GB)
as per the standard.